There are various measurement or investigation techniques which involve detecting and analyzing waveforms which have interacted with their environment. Examples of such techniques are radar techniques and acoustic techniques, both of which can be used for investigations of underground formations from within a borehole. In such techniques, one or more waveform signals are transmitted from transmitting locations so as to interact with the environment due to reflection, refraction etc., and are measured at one or more receiving locations which may be the same as the transmitting locations or separate. The measured waveforms have been changed from the transmitted form by interaction with the environment and analysis of the waveform allows properties of the environment to be obtained, for example, the presence of reflecting bodies or the nature of the medium through which the waves have passed. Since it is the spatial distribution of the measured properties which is generally of interest, the analysis must relate the information gained to some spatial reference, typically the transmitting and receiving locations. However, if there is any form of interference which is correlated with position, this can be difficult to identify in analysis. One example of such a technique which suffers from this problem is ultrasonic evaluation of cement in a cased well.
In a well completion, a string of steel casing is set in a wellbore and cement is forced into the annulus between the casing and the earth formation. The primary purpose of the cement is to provide zonal isolation of oil and gas producing layers and water bearing strata. If the cement fails to provide isolation of one zone from another, formation fluids under pressure may migrate from one zone to another, reducing production efficiency. Cement failures can occur in a variety of ways. For example, a complete absence of cement between the casing and the earth formation can occur. This is characterized as a gross cement failure and leads to rapid communication between zones intended to be isolated. Another type of failure arises when channeling occurs within the cement annulus, somewhere between the casing and the formation. There are three commonly occurring types of channels: a channel which contacts the casing is referred to as a "near channel", a channel which does not contact the casing is referred to as a "far channel" or a "buried-channel" (for a buried channel, the region between the channel and the casing is usually cement), and a channel occupying the entire space between the casing and the formation is referred to as either a "full channel" or a "traditional channel". All the channels described above may be filled with fluids such as mud or gas and all are potential threats to hydraulic isolation. Another condition which occurs, but which is not generally viewed as a cement failure, is known as micro-annulus. This condition occurs when the cement that has filled the annulus is not properly bonded to the casing resulting in a very narrow fluid-filled annulus immediately outside the casing. This annulus is very small and does not affect fluid communication between layers, effectively preserving the hydraulic security function of the cement.
A number of interfaces exist at the junctures of the differing materials within the completed wellbore. A first interface exists at the juncture of the fluid in the casing and the casing itself. A second interface is formed between the casing and a second material adjacent to the exterior of the casing, usually cement in a well-cemented situation. A third interface exists between the cement and a third material which is usually the earth formation. Imperfect cementing operations can result in a variety of interface conditions. A channel contacting the casing results in the second interface being between the casing (first material) and a fluid (second material). In this case, the third interface is formed between a fluid (second material) and the earth formation (third material) where a full channel exists. Alternatively, the third interface is formed between a fluid (second material) and the cement (third material) where a near channel exists. A channel not contacting the casing, results in the second interface being between the casing (first material) and the cement (second material) and the third interface being between the cement (second material) and a fluid (third material). Existence of an interface at the juncture of cement and fluid causes a potential lack of hydraulic isolation.
The problem of investigating the cement outside a casing with a tool located inside the casing has lead to a variety of cement evaluation techniques using acoustic energy including ultrasonic cement evaluation. U.S. Pat. No. 4,255,798 (incorporated herein by reference) describes methods and apparatus for acoustically investigating a casing in a borehole and the cement present outside the casing. Casing thickness is also determined. The techniques employ an acoustic pulse source having a frequency spectrum selected to excite a thickness resonance in the insonified portion of the casing. The thickness resonance exists as acoustic reverberations between the inner and outer walls of the casing, i.e. trapped energy. The duration of the reverberations depends on the rate of acoustic energy leaking into adjacent media. The acoustic return from the casing can be thought of in two distinct portions. The first portion appears as a large amplitude pulse (first interface echo) which represents the energy reflected from the first, fluid-steel interface, i.e., the inside surface of the casing. The second portion appears as a decaying resonance which represents the reverberating energy trapped within the casing that has leaked back into the fluid within the casing. The received acoustic pulse is then processed to determine casing thickness or to evaluate the quality of the cement bond to the casing which is typically evaluated by determining the acoustic impedance of the cement. In one example of processing, thresholds are applied to the acoustic impedance to determine if the material outside the casing at a given point is solid (cement), liquid or gas.
The presence of echoes from third interfaces present in ultrasonic waveforms gives rise to two issues in cement evaluation. The third interface echoes can interfere with the analysis of the waveforms to determine cement impedance and so give a misleading impression of the quality of the cement present and false indications of the material, e.g. indicate liquid or gas when solid is present. One method of filtering the signals to deal with these echoes is described in U.S. Pat. No. 5,274,604. The third interface echoes can also be used to identify channels in the cement which might compromise zonal isolation as is described in European Patent Application No. 0,549,419 A2.
The problem of identifying and accounting for third interface echoes in ultrasonic cement evaluation is one example of an acoustic measurement technique in which two features are present, each of which interferes with the detection of the other. In this particular example, the effects arising from the first feature, the casing, i.e. first and second interface echoes, interfere with the detection of the third interface echo. Likewise, the third interface echo interferes with the determination of cement impedance from the casing resonance. The spatial filter described in U.S. Pat. No. 5,274,604 represents one approach in dealing with this problem.
It is an object of the present invention to provide a method which can be used to analyze acoustic waveforms, particularly ultrasonic waveforms from cased wells to deal with such interferences as typified by casing echoes and third interface echoes.